Musk thistle, Carduus nutans, has been a weed pest in Oklahoma for several decades. Biological control using the musk thistle head weevil, Rhinocyllus conicus, began in 1991 and rosette weevil, Trichosirocalus horridus, in 1998. During 2000-2001, to deterimine weevil infestation levels, areas with known releases of the weevils were scouted within each county, thistle heads removed, numbers of larvae or pupa in each head counted, and percentage of each thistle head damaged was determined. The area immediately surrounding release sites in addition to other thistle infestations in the county were checked to document weevil movement. Thistle populations were assessed against past records and discussions with producers, county extension educators and road supervisors to determine if populations were reduced. Initial results appear positive. Large pasture areas previously overrun by thistle have up to 90% control. Both the head and rosette weevils are well established in release areas and have been found in adjacent thistle infestations. Presently, the decline in thistle populations appears to be related to heavier infestations of head weevil larvae and pupae from previous years. Areas with the heaviest larval/pupal counts in 2000, had the largest drop in thistle population during 2001. In addition, within certain areas of the study, rosette weevil populations have caused a dramatic reduction in both thistle population and plant vitality.
Species 1: Coleoptera Curculionidae Rhinocyllus conicus (musk thistle head weevil)
Species 2: Coleoptera Curculionidae Trichosirocalus horridus (rosette weevil)
Keywords: Musk Thistle
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA